Its not. But a lobbyist said so at some point and it gets repeated as truth.

I seem to recall I read somewhere here that SOPA would actually threaten DNSSEC. Can't find it again. Maybe I misread.

The idea was that DNSSEC would make SOPA impossible -- proper implementation of the standard would mean that if the US legislated a 'blackout' of a particular name within the US, then clients would just naturally seek out other trusted servers which would return the correct addresses.

Based on this, the assumption seemed to be that SOPA passing would render DNSSEC effectively illegal within the US. Even if not actually illegal, it would be perceived as too-risky-to-implement by large corporations; effectively killing it.